<x-flowed windows-1255>> Sounds like detonation to me. Very serious. Back off the throttle til it

stops when you hear it.

I always either back off the throttle or downshift when this happens.

To Check if this is indeed your problem, buy premium the next time you
fill
up. If the problem goes away, it is indeed detonation.

I had wondered whether I needed a higher-octane petrol but I would be quite
surprised if this is the case. I use 96 octane. Since they stopped selling
leaded petrol over here a couple of years ago, the 96 octane has had a
potassium additive added to it (I think this is what they call LRP in
England). Other options are 95, 98 and 99-octane unleaded. Surely it
shouldn't require 99 octane?